Artificially Regenerating Native Oaks in California
In California today there is evidence that several species of native oaks, including blue oak (Quercus douglasii) and valley oak (Quercus lobata), are not regenerating well in certain areas of the state. In addition to poor natural regeneration, the total acreage of these species has been depleted from residential and commercial development, agricultural conversions, and firewood harvesting. While native oaks have little commercial value, they are exceedingly important for wildlife and are highly valued for aesthetics. To ensure these species remain important components of the natural landscape of California, methods to artificially regenerate them are needed. Unfortunately, little is known about how to do this. Until recently few native oaks were produced in the State and relatively little research was directed towards growing vigorous, healthy stock or to understanding factors which influence field performance. To help develop guidelines for successfully artificially regenerating native California oaks, a variety of research projects have been undertaken during the last five years at the Sierra Foothill Range Field Station, 20 miles northeast of Marysville. These projects have focused on developing practical, low-cost techniques for producing, planting and protecting oak seedlings. Hopefully this information will promote greater success in regeneration plantings and help ensure the long-term conservation of these important species.
Several studies have examined acorn collection, handling, storage and planting practices, including the best time to collect blue oak acorns and the effect various pre-storage treatments have on germination. Results indicated that acorns can be successfully collected over a fairly wide interval, extending from late August until late October. Acorns from all harvest dates had high germination, as long as they were not allowed to dry out before storage. However, the earlier the acorns were collected, the earlier they tended to germinate. Soaking acorns for a day prior to storage had little effect. Drying acorns, however, reduced both the rate and amount of germination. A 10% reduction in moisture content resulted in almost 40% less total germination, and all acorns that lost 25% or more of their moisture failed to germinate during the 10-week test interval.
Another project examined the effect of trimming acorn radicles on field growth and survival. While radicle trimming tended to promote multi-branched root systems, it had no effect on field performance. Sowing date on the other hand, greatly influenced seedling emergence, as well as survival and height growth. Sowing in the early fall (October 10 or November 10) resulted in earlier emergence, greater survival and increased growth, compared to sowing in the late winter (March 10) for both blue and valley oaks. Early sowing gives seedlings a better chance to become established before soil moisture becomes limiting.
Producing Oak Seedlings
Several methods of growing native California oak seedlings, including both bareroot and container production, have been evaluated. This research has demonstrated that there are a variety of stock types that can be successfully propagated and outplanted. The type of stock best suited for a particular planting will depend on a host of factors including costs, seedling availability, and conditions at the site. One year old bareroot blue oak and valley oak seedlings have had high survival and vigorous growth after outplanting, as long as the roots were undercut early enough in the nursery to promote the development of a fibrous root system. Similarly, blue oak seedlings grown in a wide range of container sizes have performed well in the field. While size at outplanting was directly related to container size, by the end of the second field season, these differences all but disappeared and all container sizes grew about the same.
A study that is underway is comparing the field performance of standard one-year old blue oak container stock with that of seedlings grown for only 4 months and outplanted the spring after the acorns were collected. This study has indicated that while size at outplanting was far less for the younger stock, by the middle of the second growing season, total height was significantly greater. Since 4-month old seedlings are far cheaper to produce, they may be a preferred stock type for future regeneration programs for this species.
Seedling Planting and Protection
Many areas targeted for artificial regeneration of native oaks in California are on hardwood rangelands where dense annual vegetation and compacted soils limit recruitment success. Controlling competing vegetation through scalping, spraying or mulching can greatly improve the survival and growth of outplanted seedlings. In a study currently underway, various levels of weed control (diameter of weed-free zones around seedlings) are being compared. Preliminary results indicate that maintaining a 2-foot or larger radius weed-free circle around seedlings greatly increases soil moisture and promotes survival and growth. In this same trial, we are also comparing the field performance of seedlings protected with rigid translucent tubes to those covered with aluminum screens. During the first season, seedlings in the tubes had more flushes, grew taller, and were more resistant to attack from small sucking insects, which were able to pass through the screens.
Augering holes prior to planting can also improve field performance on certain sites by allowing seedling roots to more easily penetrate downward and obtain soil moisture unavailable at shallower rooting depths. Fertilizing seedlings with slow release fertilizer tablets at the time of planting has also resulted in large increases in both caliper and height growth on certain sites.
These studies suggest that the growth of both blue and valley oaks in a natural setting is often limited by harsh environmental conditions. By providing a more favorable environment through caging, weed control, augering and fertilization, rapid juvenile growth can be stimulated. This allows seedlings to quickly grow above the level where they are particularly vulnerable to browsing pressures, and thus helps ensure the success of regeneration plantings.
prepared and edited by John M. Harper, Richard B. Standiford, and John W. LeBlanc